Y Chen

Every morning, while their parents are still asleep, thousands of children wake up to the hum of a highway. That hum becomes the first sound of their lives, and the neighborhood where they were born becomes their first starting block. Some children begin on a level track, others on a track that tilts upward, and the pace of their biological clocks differs from the very first steps.

A drop of blood on filter paper preserves not only cells but also a record of what has been happening to the body over the past few months. That record is not in the genes but on them: the chemical marks cells attach to DNA change with food, stress, illness and age. Scientists have learned to read this archive tape to reconstruct a picture of health without repeated trips to the lab.